What is it?
Quantum entanglement is a phenomenon where two objects, though seperated in space, are referenced to each other. For example, if the spin of one particle is "up", then the spin of its entangled partner must be "down".
This implies that information can be sent faster than the speed of light which contradics classical laws of physics. However, to understand quantum entanglement, one must think in terms of the laws of quantum mechanics.
This website provides some basics of quantum mechanics and experiments that confirm quantum entanglement to help grasp the theory.